When performing horizontal fillet welding on a medium-thickness or thick plate (referred to as “medium-to-thick plate” hereinafter) by using a consumable-electrode arc welding method, it is demanded that the leg length be large (referred to as “large leg length” hereinafter) and that a wide bead be ensured. In order to ensure a large leg length and a wide bead, it is necessary to increase the amount of weld deposition per unit weld length by, for example, increasing the welding current or reducing the welding rate. However, if the amount of weld deposition is to be increased by fixing the welding current and reducing the welding rate, the production efficiency decreases, and appropriate weld penetration may be not obtained. If the amount of weld deposition is to be increased by fixing the welding current and increasing the welding rate, the temperature of a molten pool may increase, sometimes resulting in reduced viscosity. In this case, the increased amount of weld deposition and the reduced viscosity of the molten pool cause the molten pool at a standing plate side to drip toward a bottom plate due to the effect of gravitational force, sometimes resulting in inferior bead appearance, such as the leg lengths at the standing plate side and the bottom plate side being different from target leg lengths, or welding defects, such as an overlap in which the edges of the bead are simply overlapped with a base material instead of being welded thereto. Normally, fillet welding refers to triangular welding that involves welding two substantially-orthogonal surfaces at, for example, a lap joint, a T-shaped joint, or a corner joint, and horizontal fillet welding refers to fillet welding performed in a downward-facing horizontal orientation.
In order to increase the production efficiency, welding is normally performed at a high welding rate. In order to maintain the amount of weld deposition per unit weld length as the welding rate increases, the welding current has to be increased. However, because the arc force applied to the molten pool increases as the welding current increases, the molten pool directly below the arc is pushed rearward. Thus, a groove formed by the arc at the standing plate side is not supplied with molten metal and thus remains as a groove so that an undercut may form, or the shape of the solidified bead may sometimes have inferior protruding appearance.
Accordingly, the large leg length and the high welding rate in fillet welding are problematic in that inferior bead appearance and welding defects, such as undercuts and overlaps, may occur.
Normally, a method used for solving such problems involves setting the arc voltage to a high value and reducing the arc force by increasing the arc length so as to improve the bead appearance. For example, Patent Literature 1 discloses a technology for improving weld penetration into a groove wall by reducing the welding current at both weaving edges to an electric current value at which an undercut does not occur and increasing the welding rate by increasing the electric current at an intermediate weaving portion, or by increasing the welding current at both weaving edges.
The term “weaving” in this case refers to an operation for oscillating the distal end of a welding torch with reference to a welding line of the base material as the center. FIG. 12 illustrates an example of a conventional weaving operation in horizontal fillet welding. In the example shown in FIG. 12, the bottom plate is disposed horizontally, and an end surface of the standing plate is disposed on the upper surface of the bottom plate. Fillet welding is performed on the junction at which the standing plate and the bottom plate meet (an abutment angle a at the junction is 90 degrees in the example in FIG. 12). As shown in the drawing, in the conventional weaving operation, welding is performed by causing an electrode provided at the distal end of a welding torch 101 to advance while alternately moving the electrode in directions substantially orthogonal to the welding travel direction. Accordingly, in the conventional weaving operation, the welding torch 101 repeatedly moves toward the standing-plate-side weaving edge and the bottom-plate-side weaving edge so as to constantly oscillate forward in the welding travel direction.